Effects of Specific Gravity

11 February 2009

The flow equations include the variables liquid specific gravity (Gf) and gas specific gravity (Gg), which are the density of the fluid compared to the density of water (for liquids) or air (for gases).

Figure 4 [top], Figure 5 [bottom]
Figure 4 [top], Figure 5 [bottom]

However, specific gravity is not accounted for in the graphs, so a correction factor must be applied, which includes the square root of G. Taking the square root reduces the effect and brings the value much closer to that of water or air, 1.0.

For example, the specific gravity of sulphuric acid is 80 per cent higher than that of water, yet it changes flow by just 34 per cent. The specific gravity of ether is 26 per cent lower than that of water, yet it changes flow by only 14 per cent.

Figure 4 shows how taking the square root of specific gravity diminishes the significance on liquid flow. Only if the specific gravity of the liquid is very low or very high will the flow change by more than 10 percent from that of water.

The effect of specific gravity on gases is similar. For example, the specific gravity of hydrogen is 93 percent lower than that of air, but it changes flow by just 74 percent. Carbon dioxide has a specific gravity 53 percent higher than that of air, yet it changes flow by only 24 percent. Only gases with very low or very high specific gravity change the flow by more than 10 percent from that of air.

Figure 5 shows how the effect of specific gravity on gas flow is reduced by use of the square root.

Figure 4: For most common liquids, the effect of specific gravity on flow is less than ten per cent. Also, most high-density liquids such as concentrated acids and bases usually are diluted in water and – consequently – the specific gravity of the mixtures is much closer to that of water than to that of the pure liquid.

Figure 5: For common gases, the specific gravity of the gas changes flow by less than ten per cent from that of air. And just as with liquids, gases with exceptionally high or low densities often are mixed with a carrier gas such as nitrogen, so that the specific gravity of the mixture is close to that of air.


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